Optical components that switch and/or attenuate selected channels (hereinafter wavelength selective switch, WSS) in wavelength-division multiplexed (WDM) communications are useful in constructing optical mesh networks and reconfigurable optical add/drop multiplexers (ROADM). With reference to FIG. 1, a typical WSS is depicted by 100. WSS 100 is implemented in a hybrid planar lightwave circuit (PLC) 120 and free-space optics configuration 160 (described in previously referenced U.S. patent application Ser. No. 10/930,382 and the article “Compact, wavelength-selective 1×2 switch utilizing a planar lightwave stack and a MEMS micromirror array,” by D. M. Marom and C. R. Doerr, published in Optical MEMS Conference in August 2004). WSS 100 includes dispersing means PLC 120 for spatially dispersing the optical signal (e.g., a WDM signal) from the input fiber 115 and projecting the dispersed light through a lens 135 onto a spatial light modulating (SLM) device 140. The SLM device 140 is typically comprised of an array of individual pixels, where each pixel is assigned to a particular wavelength channel or band of channels and can either attenuate the optical signal (for an optical channel blocker) or switch the optical signal onto an output fiber (for a WSS). The SLM technology commonly used for such optical components is based on either liquid crystals or microelectromechanical system (MEMS)-based micromirrors.
While each channel of the optical signal is directed to an assigned pixel, undesirably, some spectral components of the channel may fall on the edge of the pixel, especially in systems with high spectral efficiency. The diffraction phenomena from the edge can result in some deleterious effects, such as coupling the light to undesirable locations. (See article “Attenuated mechanism effect on filter shape in channelized dynamic spectral equalizers,” by S-H. Oh and D. M. Marom, published on Jan. 1, 2004 in Vol. 43, No. 1, APPLIED OPTICS) Hence beams of light of frequency components that are incident on the interpixel gap region represent a special challenge to the optical design of the WSS. Furthermore, when SLM 140 is a MEMS structure, each pixel's micromirror will have a finite curvature. This curvature will cause the reflected light to reflect in slightly different directions depending on small changes in wavelength (see article “Effects of mirror curvature in MEMS micromirror based wavelength-selective switches” by D. M. Marom et al, published in 2003 LEOS annual meeting, Tucson, Ariz., October 2003.). Therefore strict bounds are set on the minimum acceptable micromirror curvature, which represents a challenge for metalized mirrors due to stresses and coefficient of thermal expansion mismatches.
What is desired is an improved WSS design, which solves both the problem of light falling on pixel edges and finite micromirror curvature.